1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine or a printer.
2. Description of the Related Art
Up to now, a contact charging device, which has a system in which a voltage is applied to a charging member abutted against a photosensitive member to charge the photosensitive member, is put into practical use for an electrophotographic image forming apparatus because of having an advantage such as low ozone and low power. In particular, a device having a roller charging system using a charging roller as the charging member is preferred in terms of charging stability and is used widely.
In the contact charging device having the roller charging system, when a voltage (charge starting voltage Vth) equal to or higher than a fixed value is applied to the charging roller, a surface potential of the photosensitive member starts rising, and after that, increases linearly with an applied voltage at a slope of 1. That is, in order to obtain the surface potential (Vd) of the photosensitive member necessary for electrophotography, it is necessary to apply a DC voltage of Vd+Vth to the charging member.
Here, in a DC charging system, as a method of enhancing uniformity of the surface potential of the photosensitive member, the following conventional technology is proposed. Specifically, the photosensitive member is primarily charged to a potential equal to or higher than a non-image section potential (Vd) necessary for image formation temporarily, and a photosensitive member potential is exposed to weak light emitted from an exposure device (post-exposure device) located in a position after the primary charging and before developing, to thereby attenuate (lower) the surface potential. This is a potential control method of producing a target non-image section potential (Vd).
Here, Japanese Patent Application Laid-Open No. 2008-8991 proposes a method of setting a high primary charging potential based on a degree of non-uniformity (transfer memory) of the photosensitive member potential after the transfer, which is caused by presence/absence of a toner image and a transfer field strength, so as to increase a difference between the primary charging potential and the non-image section potential (Vd).
On the other hand, in the DC charging system, the charge starting voltage Vth changes depending on a photoconductive layer film thickness of the photosensitive member, and hence the non-image section potential (Vd) rises when a photosensitive member film thickness is reduced due to wear of the photosensitive member. Therefore, Japanese Patent Application Laid-Open No. 2002-296853 proposes a method of maintaining latent image potential setting at a constant level by calculating the photosensitive member film thickness from information on any one of the number of supplied sheets, the number of revolutions of the photosensitive member, and a charging voltage application period to control the exposure amount. According to this method, an image density, a line width, and a tone reproduction can be stably reproduced by changing ranges of a maximum light amount for forming an image section potential (Vl) and a minimum light amount for forming the non-image section potential (Vd) depending on the calculated photosensitive member film thickness. According to those conventional technologies, in a color image forming apparatus including a plurality of photosensitive members, a non-image section exposure amount for each of the photosensitive members is controlled based on the photosensitive member film thickness. With this control, even when voltages having a common value are applied to charging rollers configured to charge the respective photosensitive members, it is possible to obtain a constant non-image section potential (Vd). In addition, an image section exposure amount for forming the image section potential (Vl) is controlled based on the photosensitive member film thickness at the same time. With this control, commonality can be achieved among charging voltages for the plurality of photosensitive members and among developing voltages applied to developing devices configured to develop electrostatic latent images on the respective photosensitive members, which can downsize an apparatus and lower cost therefor.
However, in an image forming apparatus configured to form the non-image section potential (Vd) by performing non-image section exposure (background area exposure), the photosensitive member is repeatedly exposed over long-term use, to thereby cause reduction in sensitivity due to light-induced fatigue, which may cause an image failure such as a reduction in the image density. In other words, from the viewpoint of the reduction in the sensitivity of the photosensitive member, it is preferred that a background area exposure amount be as small as possible.
However, in a so-called inline color image forming apparatus in which the plurality of photosensitive members are arranged from upstream to downstream along a rotational direction of an intermediate transfer member, transfer memory exhibited by the photosensitive member of a downstream station in the primary transfer position may differ in magnitude depending on the image density in an upstream station. Therefore, when the image density is high in the upstream station, the difference between the primary charging potential and the non-image section potential (Vd) in the downstream station is set large. In other words, in order to obtain the primary charging potential higher than the non-image section potential (Vd) necessary for the image formation, the charging voltage is set high, and the background area exposure amount is set large.
Further, in a compact color image forming apparatus using a charging voltage common to the plurality of photosensitive members and a developing voltage common to a plurality of developing devices opposed to the plurality of photosensitive members, it is necessary to maintain the non-image section potential (Vd) at a constant level in all stations. Therefore, the background area exposure amount is set large in the station having the photosensitive member reduced in film thickness during use.